Aluminum coating

ABSTRACT

This invention relates to an improved method of providing a color coating on the surface of aluminum products as a protection against corrosive action and for a decorative coating, the method comprising treating the aluminum products with a solution containing an aldehyde and an alkali.

United States Patent [191 Venlrata s.

[ Dec.4,1973

[52] US. Cl. 148/627, 117/66, 117/132 BF [51] Int. Cl. C231 7/00 [58] Field of Search 148/6, 6.14, 61.27;

[56] References Cited UNITED STATES PATENTS 2,662,034 12/1953 Mason et a1. 117/132 BF 3,156,670 ll/l964 Soldatos 117/132 BF 3,274,032 9/1966 Every et al.. 148/614 R 3,162,547 12/1964 Kendall 148/614 R X Primary Examiner-Edward G. Whitby AttorneySanford Astor [57 ABSTRACT This invention relates to an improved method of providing a color coating on the surface of aluminum products as a protection against corrosive action and for a decorative coating, the method comprising treating the aluminum products with a solution containing an aldehyde and an alkali.

5 Claims, No Drawings I ALUMINUM COATING Decorative and protective oxide coatings on aluminum have long been made by anodizing in electrolytes consisting of an aqueous solutions of sulphuric acid and other acids. Such processes usually require electrical energy, sometimes controlled refrigeration, and costly electrical equipment. In recent years some coating processes were developed which do not require refrigeration but still need electrical energy and too many controls.

The known processes mentioned above are not satisfactory in respect to the degree of protection which they afford. The standard test for resistance to corrosion is made by the application to the aluminum surface of saltwater spray of percent salt content at a temperature of 95 F., and a relative humidity of 95-100 percent. The resistance life of the treated aluminum is measured by the duration of the elapsed time before the surface is dulled, stained and later pitted by the salt spray. it is standard practice to measure the resistance of the treated surface of aluminum as 1 year for each 24 hour period of application of the salt spray without evidence of corrosion. Under this test the aluminum surface coatings produced under the above known methods, vary widely in resistance life from five or less years to ten years. Most of the known methods produce coatings which hold upfor greatly shorter periods than the maximum of ten years.

Chemically pure aluminum has little commercial use since it has a tensile strength of only about 7,000 psi. However, with the addition of small amounts of iron or silicon for examply the tensil strength is doubled. Certain alloys of aluminum have tensil strengths as high as 100,000 psi. These products also possess increased hardness, ductility and machineability compared to pure aluminum.

Commercial forms of aluminum and many of its useful alloys also acquire naturally formed aluminum oxide coatings; however, they are subject to more rapid corrosion possibly due to galvanic action within the alloy or to entry of corrosive agents through breaks in the coating. A protective coating of a more effective character than the naturally formed aluminum oxide is needed to protect as well as to beautify the aluminum. The object of this invention is to provide a coating but without using any amount of electrical energy. It is also the object of the invention to make the process free of any organic solvents. Further the object of the invention is to impart color to the surface in an atmosphere of chemical reduction as well as to impart a fade resistant and corrosion resistant coating.

SUMMARY OF THE INVENTION The process of this invention is a chemical treating process having several steps, described fully hereinafter.

In the first step of the process, the aluminum stock material, whether aluminum plate, aluminum extrusion or other stock, or the aluminum fabricated product which is to be treated, is immersed or otherwise brought into contact with a cleansing liquid for the re moval of the superficial layer on the aluminum of dirt, oil and grease. This is a preferably nonionic detergent in a weak solution, as for instance rfipercent to 20percent which by its wetting and emulsifying action thoroughly cleans and'carries away the foreign bodies on the surface of the metal, preventing their being redeposited on the metal which in the bath, thus readying the aluminum pro'ductfor action ofthe chemicals of the process upon the oxide film adhering to its surface. This bath is preferably at room temperature, but can be operated at up to F., to meet the needs of certain fabricated products.

In the next step of the process the aluminum is dipped in a bath of a solution of sodium hydroxide, of preferably 2 to 20 percent by weight, for a period of 1 to 6 minutes for example, and at a temperature of from ambient to 160 F. Here the naturally present and not cleaned surface layer of aluminum oxide is at least partially converted into sodium aluminate, which is thereby freed from the surface of the aluminum. If impurities are present in the surface oxide, as they usually are, in this step of the process the aluminum surface becomes slightly discolored, and the surface of the metal usually shows a slight etching.

In the next step the aluminum product is subjected to a cold water rinse in a tank, provided with a steady supply of clean water at ambient temperature. This rinse removes the sodium hydroxide stopping its action upon the aluminum surface.

The aluminum product is then immersed in a bath in an acid resistant tank, which is a 2 to 20 percent solution in water of a material which for brevity of identification may be referred to as Recobright-D for a period of 2 to 10 minutes at room temperature. Recobright-D is made by dissolving in water from 1 to 3.5 parts of sodium dichromate'to 2 to 6 parts of sodium acid fluoride with 10 to 20 parts of sulphuric acid, by weight. This bath has a chemical reation on the oxidation products freed in the bath of the second step of the process, the product of this reaction being removed from the aluminumproducts and dissolved in the acid solution.

The aluminum product is then subject to a cold water rinse.

The aluminum product is then dipped in a mild hydroxide solution such as potassium hydroxide of about 0.5 to 1 percent at 130 to F. for from 1 to 5 minutes, to produce fresh aluminate on the surface of the aluminum product.

In the next and most distinctive step of the process, the aluminum product is immersed in a bath which is an aqueous solution of an organic compound with a chemical formula RCHO in which R is hydrogen or any other chemical group. This would be described as an aldehyde. The solution of this mixture should be at a strength of approximately 0.3 to 25 percent by weight depending somewhat on small variations in the formula of the aldehyde which becomes the solute.

Strengths of 0.3 to 25 percent are effective but 10 percent is preferred. This bath is preferably room temperature to 150 F. The chemical reaction takes place faster in an alkaline pH factor of less than 1 l.

Aldehydes with low molecular weights such as formaldehyde are more effective than the aldehydes with a high molecular weight such as propionaldehyde. Preferably an aldehyde with 6 carbon atoms in the molecule is most effective.

The process is effective with aldehydes or raw materials which produce aldehydes, such as ethylene glycol.

After 5 minutes to 1 hour in the aldehyde bath at 80 degrees to 150 F. an extremely thin, almost monomolecular film of a complex salt is formed on the surface of the aluminum.

The surface coating created by the reaction taking place in this step is a chemical compound of a complex nature. It is believed the coating is composed of Al(C- H OH) although the exact nature of the compound is not known.

When removed from the tank of aldehyde solution, the aluminum is rinsed in cold water.

The aluminum product may then be rinsed in an oxidizing agent containing at least 2 oxygen atoms in the molecule. This step is optional in case a greyish color develops on the aluminum due to impurities in the aluminum.

The aluminum product with its treated surface is then dipped into a resin solution such as acrylic and melamine resins common in the art. Such resins are manufactured by Rohm and Haas Company and are known as Acrysol WS-24 and U-Formite MM83. The resin solution has a pH of about 9 to about 11 and the aluminum product is dipped for a l to 2 minute period.

The aluminum product is then baked in an oven at a temperature of between about 250 F. and 400 F. for a period of about to 20 minutes; 350 F. is preferred for best results. This baking causes a rapid curing of the coated metal product.

In the aldehyde treatment step the addition of iron salts and/or ammonium salts will give the surface a darker color. The use of ferric ammonium oxalate plus nitric acid will give ferric nitrate that also helps as a rust inhibitor.

The aldehyde treatment has been accomplished in many tests which the following is an example:

Water 98.5 cc

Formalin 37% 1.0 cc

Dimethyl Ethanolamine 0.5 cc

The aluminum after being prepared as previously described was treated in the above solution which constituted 0.37 percent formaldehyde at 100 F. for 20 minutes.

This treatment was followed by a plastic coat by dipping the aluminum for 2 minutes at room temperature in the following solution:

Acrylic emulsion Acrysol WS-24 (35%) 42.50 Water 36.70 Dimethyl ethanolamine 4.25 lsopropanol l 1.70 Methoxy melamine resin MM-83 (80%) 4.85 100.00

The isopropanol acts as a solvent for the melamine resin.

The aluminum was then baked at 300 F. for minutes. An excellent bronze colored resistant coating was formed on the aluminum which resisted any damage by salt spray when sprayed for 1,800 hours.

Tests were also run giving excellent results using the following aldehyde baths:

Water 1,000 cc Polyethylene glycol 6 cc Hydrogen peroxide 1 cc Sodium hydroxide 0.01 cc The aluminum was treated for 20 minutes at F. and then plastic coated as in Example 1.

Water 299 cc Bytyraldehyde 1 cc Dimethyl ethanolamine 3 cc The aluminum was treated for 30 minutes at 140 F., then plastic coated as in Example 1.

Water 299 cc Glutaraldehyde 1 cc Dimethyl ethanolamine 3 cc The aluminum was treated for 50 minutes at 140 F., then plastic coated as in Example 1.

The coating created during the process described is a layer of material which will stand up under 1,800 hours of salt spray, indicating a useful life of more than 10 years, having high reflective power (a quality of great utility when the aluminum is used in heaters or lighting equipment). It also gives the surface of the aluminum product on which it has been formed a beautiful bronze color greatly increasing the attractiveness of the metal. The coating is also possessed of high durability.

Having thus described the invention, it is desired that the invention be limited only by the scope of the claims.

I claim:

1. The process for treating aluminum comprising cleaning the aluminum of foreign substances adhering thereto, treating the aluminum to free surface oxides from the surface of the aluminum and coating the aluminum with a thin layer of adhering material formed by treating the aluminum with an aldehyde in an alkaline medium.

2. The process of claim 1 in which the aldehyde is formaldehyde.

3. The process of claim 1 in which the treatment material consists of 0.37 percent formaldehyde, 0.5 percent dimethyl ethanolamine and 99.13 percent water.

4. The process of coating aluminum products with a thin layer of adhering material formed by the chemical action upon the aluminum comprising:

a. Cleaning foreign substances adhering to the aluminum,

b. Freeing surface oxides from the aluminum product by treatment with sodium hydroxide,

c. Treating the aluminum with an aldehyde in an alkaline medium at from 80 to F. for from 5 minutes to 1 hour,

(1. Treating the aluminum with a resin,

e. Baking the aluminum at from 250 to 400 F. for

from 10 to 20 minutes.

5. The process of claim 4 in which the aldehyde is formaldehyde. 

2. The process of claim 1 in which the aldehyde is formaldehyde.
 3. The process of claim 1 in which the treatment material consists of 0.37 percent formaldehyde, 0.5 percent dimethyl ethanolamine and 99.13 percent water.
 4. The process of coating aluminum products with a thin layer of adhering material formed by the chemical action upon the aluminum comprising: a. Cleaning foreign substances adhering to the aluminum, b. Freeing surface oxides from the aluminum product by treatment with sodium hydroxide, c. Treating the aluminum with an aldehyde in an alkaline medium at from 80* to 150* F. for from 5 minutes to 1 hour, d. Treating the aluminum with a resin, e. Baking the aluminum at from 250* to 400* F. for from 10 to 20 minutes.
 5. The process of claim 4 in which the aldehyde is formaldehyde. 